1. Field of the Invention
The invention relates to mobile communications comprising, for example, data, voice and multimedia. Particularly, the invention relates to a method for performing a handover in mobile communication system.
2. Description of the Related Art
Fourth generation mobile communication systems are being developed as a further step of evolution since the introduction of 2G and 3G mobile communication systems. A notable example of a 2G mobile communication system is the Global System of Mobile Communication (GSM) standardized by European Telecommunication Standards Institute (ETSI), which provides a digital circuit switched data using a Time Division Multiple Access (TDMA) based radio interface. GSM achieves downlink data rates up to 64 kbps with circuit switched data and up to 144 kbps with packet switched data using the General Packet Radio Systems (GPRS) technology also standardized in ETSI. From the 3G side of the family of mobile communication systems most notable example is the Wideband Code Division Multiple Access (WCDMA) radio technology based Universal Mobile Communications System (UMTS). UMTS supports packet switched data rates up to 384 kbps and up to 4 Mbps with High-Speed Downlink Packet Access (HSDPA). The fourth generation mobile communication systems, also referred to as the Next Generation Mobile Networks (NGMN), aim to provide downlink data rates up to 100 Mbps and uplink data rates up to 50 Mbps. The downlink data rates basically enable the receiving of High Definition Television (HDTV) signals. A 4G mobile communication system is being standardized by the 3G Partnership Project (3GPP) under the title Long-Term Evolution (LTE). The LTE architecture, referred to also as the Evolved Packet System (EPS), comprises an Evolved Packet Core (EPC) and an Evolved UMTS Radio Access Network (E-UTRAN). LTE relies on the Orthogonal Frequency Division Multiplexing (OFDM) radio technology. EPC also supports a variety of radio access technologies in addition to OFDM based E-UTRAN. EPC supports alternative radio access technologies such as WLAN and WiMAX. It is also a necessary objective for the EPC to support legacy Radio Access Technology (RAT) based access networks such as GSM-EDGE Radio Access Networks (GERAN) and UMTS Radio Access Networks (UTRAN). It has been postulated that there will be a long transition period in the deployment of full coverage using E-UTRANs or alternative RAT based RANs. Therefore, the support of legacy RATs is essential. There is also going to be interworking between the EPC and legacy core networks such the Circuit Switched (CS) GSM core network and the CS domain of the UMTS core network.
Reference is now made to FIG. 1, which illustrates an Evolved Packet System (EPS) in prior art. In FIG. 1 there is illustrated an Evolved Packet System (EPS) 100. In Evolved Packet System 100 there is an Evolved UTRAN (E-UTRAN) 102. E-UTRAN 102 communicates with Evolved Packet Core (EPC) 104. Evolved Packet Core 104 communicates with IP Network 116. There is also a UTRAN 106, which communicates with Packet Switched Core 108. Packet Switched Core 108 communicates with an IP Network 116. There is also a GERAN 110 which communicates with Circuit Switched (CS) core 112. CS core 112 communicates with Public Switched Telephone Network (PSTN) 114, which may represent any circuit switched network.
There is also illustrated a UE 101, in other words, a mobile station. UE 101 may comprise a smart card such as, for example, a USIM or a SIM.
In E-UTRAN 102 there are illustrated three eNodeBs, namely an eNodeB 120, an eNodeB 122 and an eNodeB 124. ENodeBs 120-124 have a signaling plane connection to a Mobility Management Entity (MME) 130 as illustrated with lines encircled by oval 180. ENodeBs 120-124 have user plane connections to an S-GW 132, as illustrated with lines encircled by oval 181. In EPC 104, there is a Mobile Management Entity (MME) 130. S-GW 132 is connected to a Packet Data Network (PDN) Gateway (P-GW) 134, as illustrated with the line 182. EPC 104 is connected to IP Network 116 as illustrated with line 183.
In UTRAN 106 there are illustrated two nodeBs, namely a nodeB 140 and a nodeB 142, which communicate with a Radio Network Controller (RNC) 144. The lines representing user plane and signaling plane connections from nodeB 142 to RNC 144 are encircled with oval 185. The user plane and signaling plane connections from nodeB 140 to RNC 144 are encircled with oval 186.
In PS Core 108, there is an SGSN 150 and a GGSN 152. The user plane and signaling plane connections from RNC 144 to SGSN 50 are illustrated with oval 187. The user plane and signaling plane connections between SGSN 150 and GGSN 152 are illustrated with oval 188. The GGSN 152 has an Access Point (AP) to IP network 116.
In GERAN 110 there is a base station 160 and a base station controller 162. The signaling plane and user plane connections are encircled oval 190. In circuit switched core 112 there is an MSC 170. In practice MSC 170 may comprise an MSC server and a media gateway and a signaling gateway. The user plane and signaling plane connections from BSC 162 to MSC 170 are encircled with oval 191. The user plane and signaling plane connections from CS Core 112 to PSTN 114 are encircled with oval 192.
An eNodeB such as eNodeB 120 acts as a base station in an EPS. An eNodeB performs radio resource management comprising radio bearer control radio admission control, connection mobility control and dynamic allocation of resources to UEs. An eNodeB also performs IP header compression and encryption of user plane data traffic. An eNodeB selects an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE. An eNodeB also performs mobility management signaling with an MME. It routes a user plane data towards a serving gateway. An MME performs mobility management related functions. It performs tracking area list management, selects an S-GW and a P-GW for a UE. It selects MME in association with handovers. A serving gateway acts as local mobility anchor point for inter eNodeB handover. It performs packet routing and forwarding towards eNodeBs. A serving gateway also performs E-UTRAN idle mode downlink packet buffering an initiation of network trigged service requests. It also performs transport level packet marking in the uplink and the downlink directions. It also performs accounting and charging. A P-GW performs UE IP address allocation. It performs per user based package filtering by the package inspection. It performs transport level package marking in the downlink. It generally acts as an interface towards an external IP-NW such as the internet or an intranet.
EPC provides only a packet switched domain wherein calls may only be established as multimedia sessions using, for example, an IP Multimedia Subsystem (IMS). A problem with the existing packet switched EPC architecture is that call establishment may take up to several seconds. Another related problem is that the EPC architecture regarding the interworking with CS networks is complicated, which may contribute to a further delay.
In order to support CS calls and CS data bearers without necessitating User Equipments (UE) to camp exclusively on GERAN or UTRAN provided cells, it would be beneficial to be able to perform fallbacks to GERANs or UTRAN to establish CS calls or CS data bearers. It may be estimated that call establishment is initially faster through a legacy CS core network. Furthermore, in the case of mobile terminated CS calls initiated via the Public Switched Telephone Network (PSTN) there is no need to perform interworking through a VoIP gateway system. Furthermore, it is necessary to be able to perform a fallback to a correct Mobile Switching Center (MSC) which serves the current location area of the UE and to avoid unnecessary call attempts via a wrong MSC.